The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Vehicles may be provided with at least one catalytic converter for reducing emission (EM) contained in an exhaust gas. The exhaust gas flowing out from an engine through an exhaust manifold is driven into a catalytic converter mounted at an exhaust pipe and is purified therein. After that, noise of the exhaust gas is decreased while passing through a muffler and then the exhaust gas is emitted into the air through a tail pipe. The catalytic converter purifies the EM contained in the exhaust gas. In addition, a particulate filter for trapping particulate matter (PM) in the exhaust gas is mounted in the exhaust pipe.
A three-way catalyst (TWC) is one type of the catalytic converter and reacts with hydrocarbon (HC) compounds, carbon monoxide (CO) and nitrogen oxides (NOx), which are harmful components of the exhaust gas, to remove these compounds. The TWCs are mainly installed in gasoline vehicles, and Pt/Rh, Pd/Rh or Pt/Pd/Rh systems are used as the TWCs.
A lean-burn engine among gasoline engines improves fuel efficiency by burning a lean air/fuel mixture. The lean-burn engine burns the lean air/fuel mixture, so air/fuel ratio (AFR) of the exhaust gas is also lean. However, when the AFR is lean, the TWC slips the NOx without sufficiently reducing all of the NOx contained in the exhaust gas. Accordingly, a vehicle equipped with the lean-burn engine may include a selective catalytic reduction (SCR) catalyst for purifying the NOx slipped from the TWC. The SCR catalyst used in the vehicle equipped with the lean-burn engine may be a passive type SCR catalyst.
When the AFR is rich, the TWC may reduce the NOx to produce NH3 and the NH3 generated in the TWC is stored in the passive type SCR catalyst. When the AFR is lean, the passive type SCR catalyst purifies the NOx contained in the exhaust gas using the stored NH3.
The lean-burn engine equipped with the TWC and the passive type SCR catalyst may adjust the AFR to be rich by increasing fuel for a predetermined duration in order to store a sufficient amount of the NH3 in the passive type SCR catalyst. If the amount of the NOx discharged from the lean-burn engine increases, the number and duration where the lean-burn engine operates at the rich AFR also increase. Therefore, fuel economy may be deteriorated.
To decrease deterioration of fuel economy, the amount of the NH3 generated at the rich AFR must be increased. In this case, the duration for which the rich AFR is maintained can be reduced, thereby suppressing deterioration of fuel economy. An ammonia production catalyst (APC) may be added to a downstream of the TWC to increase the amount of the NH3 produced at the rich AFR. The APC can store the NOx contained in the exhaust gas at the lean AFR and produce the NH3 using the stored NOx and the NOX contained in the exhaust gas at the rich AFR. Therefore, the APC can produce more NH3 than the TWC does at the rich AFR.
However, since the APC contains components capable of storing the NOx, if the engine is operated at the rich AFR in a state that a temperature of the APC is low, nitrous oxide (N2O) may be generated in the APC. Therefore, if the temperature of the APC is low when conversion to the rich AFR is desired or required, we have discovered that the APC should be heated to a predetermined temperature.
In addition, we have discovered that the CO and the HC may be slipped from the TWC at the rich AFR. The CO and the HC slipped from the TWC may not be purified but be exhausted to an exterior of the vehicle. Therefore, an additional catalytic converter or control for reducing the CO and the HC slipped when the AFR is rich in order to produce NH3 may be desired.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.